Measurements were made of alpha 2u content of rough and smooth endoplasmic reticulum as well as Golgi apparatus. Specific activity and time of appearance of 14C-labeled amino acids in alpha 2u were determined in RER, SER, and GA. Alpha 2u followed much the same time sequence as serum albumin. Renal reabsorption of alpha 2u was measured in rats maintained on 0%, 20%, and 50% casein diets. Protein reabsorption on 0% casein was almost complete; on 50% casein it decreased to compensate for the elevated excretion. The rate of alpha 2u input by the liver was determined on rats placed on 20% casein diet after having been depleted of protein for 10 days. Although the serum levels were restored in 2-4 days, 6 days were needed to restore input; urinary excretion was delayed 6-8 days. Urinary albumin, alpha 2u and total protein were determined in rats given puromycin aminonucleoside and sodium maleate. PAN increased the excretion of albumin and toal urinary protein but not alpha 2u; MAL, which affects protein reabsorption, increased the urinary excretion of all three. Preliminary studies indicate that alpha 2u may play a role in the stimulation of spermatogenesis.